Conventionally, generally, catalysts of a heterogeneous system and catalysts of a homogenous system are known as a catalyst for hydrogenation to be used in the step of hydrogenating an olefinic compound.
Although the former catalysts of the heterogeneous system are widely used industrially, the catalysts of the heterogeneous system generally have lower activity than that of the latter catalysts of the homogenous system and have the problem of being economically inefficient, because the catalysts of the heterogeneous system are required to be used at a large amount for a desired hydrogenation reaction, and the reaction is at a high temperature and a high pressure.
On the other hand, since a hydrogenation reaction using the latter catalysts of the homogenous system usually proceeds in the homogenous system, there are features that the catalysts of the homogenous system have a higher activity, require a lower amount of used catalyst, and are capable of hydrogenation at a lower temperature and a lower pressure as compared with the catalysts of the heterogeneous system. However, the catalysts of the homogenous system have the drawbacks that catalyst preparation is complex, the stability of the catalyst itself cannot be said to be sufficient, reproducibilities are poor, and side reactions tend to occur. Further, the catalysts of the homogenous system also have the problem that sufficient activity of hydrogenation is not obtained when hydrogenating an alkyl-substituted olefinic unsaturated double bond having a steric hindrance.
In view of the above, there is currently a strong need for the development of a catalyst for hydrogenation which has a high activity and which can be handled easily.
In addition, for a polymer containing an olefinic unsaturated double bond, although the unsaturated double bond is advantageously utilized in vulcanization and the like, due to this double bond, the polymer also has the drawbacks regarding poor stability, such as heat resistance and oxidation resistance. This drawback regarding poor stability is substantially improved by hydrogenating the polymer to remove the unsaturated double bonds in the polymer chain.
However, when hydrogenating the polymer, as compared with hydrogenating a low-molecular-weight compound, the polymer is more easily affected by the viscosity of the reaction system, steric hindrance of the polymer chain and the like, which makes hydrogenation more difficult. In addition, there is the drawback that it is very difficult to physically remove the catalyst and substantially impossible to completely separate the catalyst, after hydrogenation has finished.
As described above, there has long been an issue for obtaining a catalyst for hydrogenation that is economically efficient not need to be used in a large amount, has a high storage stability, can exhibit sufficient hydrogenation activity even when hydrogenating olefinic unsaturated double bonds having a steric hindrance, and can be easily separated and removed after hydrogenation.
In consideration of such an issue, Patent Literature 1 and 2 disclose a method for hydrogenating an olefin compound using a combination of a specific titanocene compound and alkyl lithium. Patent Literature 3 and 4 disclose a method for hydrogenating an olefinic unsaturated (co)polymer using a combination of a metallocene compound and organic aluminum, zinc, and magnesium. Patent Literature 5 and 6 disclose a method for hydrogenating an olefinic unsaturated group-containing living polymer using a combination of a specific titanocene compound and alkyl lithium.
Further, Patent Literature 7 discloses a method for hydrogenating the olefinic double bond in an olefinic unsaturated double bond group-containing polymer using a combination of a specific titanocene compound and alkoxy lithium. It is noted that this method also requires an expensive organic metal compound in addition to the alkoxy lithium as a reductant.
In addition, Patent Literature 8 discloses a method for hydrogenating an olefinic unsaturated double bond containing polymer using a combination of a specific titanocene compound, an olefin compound, and a reductant.
Still further, Patent Literature 9 discloses a method for hydrogenating an olefin compound using a combination of a metallocene compound having a pentamethylcyclopentadienyl group in which all five hydrogen atoms of a cyclopentadienyl group have been substituted on with a methyl group, and a reducing agent.
Still even further, Patent Literature 10 and 11 disclose a method for hydrogenating an olefin compound using a catalyst composition for hydrogenation which includes a specific titanocene compound, a reductant, an olefinic unsaturated double bond-containing polymer, and a polar compound.
Further, Patent Literature 12 discloses a method for hydrogenating an olefin compound using a catalyst composition for hydrogenation which includes a specific metallocene compound and a compound selected from a conjugated diene monomer, an acetylene compound, and an acetylene monomer.